1. Field of the Invention
The invention relates to a UV-curable adhesive composition for a semiconductor chip mounting process and also to said mounting process.
2. Description of the Related Art
In the course of the development of semiconductor devices, the performance of said devices is being increased by progressive miniaturization of the devices together with the structures contained thereon. If technical reasons require a certain spatial size for a device which exceeds that of an individual device, a large device is often assembled in modular construction from smaller units. Depending on the complexity of the circuits or the size of the structures contained thereon, a spatially precise relative arrangement of the individual modules with respect to one another or to the substrate or carrier may be necessary in assembling the individual modules, which may be similar or even different. The maximum permissible error tolerances may in that case be in the region of a few .mu.m and are often even less.
WO 89/08896, for example, describes a nonmechanical printer functioning on the electrophotographic principle. Its character generator contains a multiplicity of light sources which are arranged in an exposure line. These are normally light-emitting diodes (LEDs) which are, for example, integrated in two rows of 64 LEDs each on a monolithic chip. The chips are in turn assembled on individual modules from which the exposure line can in turn be assembled in accordance with the paper format to be printed. The achievable printing quality depends on the fineness of the screen, that is to say on the spacing of the individual LEDs on the chip. High-resolution character generators currently achieve a print screen of 600 dots per inch (dpi). With an exposure line 450 mm wide, this corresponds, for example, to a quantity of over 10,000 individual LEDs which are adjacently arranged in two rows with gap offset. In order not to adversely affect the printing quality at this printing density, it is necessary to maintain an error tolerance of .+-.2 .mu.m when assembling the individual chips on a module. For this purpose, the individual chips have to be precisely aligned during mounting on the module before the final fixing. After the alignment, the position of the chips must no longer change during the fixing. For the soldering process normally used for this purpose, this requires, however, extensive precautions since, as is known, the solder fuses during soldering but sliding of the chips, in particular, should be avoided.
Although chips can be fixed with positional accuracy by gluing, a high heat loss has to be dissipated during operation because of the high power density of the integrated chips, and this is only possible via a solder joint.
German Published Application 38 08 667 proposes gluing the chips onto a temporary carrier in an arrangement which is mirror-inverted with respect to the finished module, the precise alignment already being carried out. In this fixed form, the chips are then soldered onto the actual module. After completion of the soldering process, the adhesive bond is released and the temporary carrier consequently removed. However, it has hitherto not been possible to find for this process a suitable adhesive which both makes possible a rapid fixing of the chips on the temporary carrier, the fixing being resistant in the soldering process, and can also be removed again completely after the soldering process.
From IBM Technical Disclosure Bulletin 13, (1971), page 3652, it is known to glue a semiconductor wafer to a temporary carrier with the aid of a photoresist, to divide the wafer into individual chips, to fix the bonding of serviceable chips by means of UV light and, finally, to release the fixed bonds again with the aid of a solvent.
EP-0 142 783 discloses a process for mounting semiconductor devices on a substrate, in which the devices are first glued to the substrate with the aid of a thermally curable adhesive containing styrene polymers. After thermal curing, the devices are soldered on and, finally, the adhesive is removed again with the aid of a solvent.